libbb: code shrink and speed up index_in_strings()

Rewrite index_in_strings() to replace calls to strcmp()/strlen().
With this change searching for valid names in the applet_names
array (for example) is 40% faster.

The code has to assume the strings aren't sorted, so will always scan
the entire array when presented with an invalid name.

function                                             old     new   delta
index_in_strings                                      63      56      -7
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-7)               Total: -7 bytes

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

libbb/compare_string_array.c

@@ -63,13 +63,19 @@ int FAST_FUNC index_in_str_array(const char *const string_array[], const char *k
 
 int FAST_FUNC index_in_strings(const char *strings, const char *key)
 {
-	int idx = 0;
+	int j, idx = 0;
 
 	while (*strings) {
-		if (strcmp(strings, key) == 0) {
-			return idx;
+		/* Do we see "key\0" at current position in strings? */
+		for (j = 0; *strings == key[j]; ++j) {
+			if (*strings++ == '\0') {
+				//bb_error_msg("found:'%s' i:%u", key, idx);
+				return idx; /* yes */
+			}
 		}
-		strings += strlen(strings) + 1; /* skip NUL */
+		/* No.  Move to the start of the next string. */
+		while (*strings++ != '\0')
+			continue;
 		idx++;
 	}
 	return -1;